This invention relates to a recording system comprising a control apparatus and a recording apparatus which changes recording density upon receiving a recording density changeover command from the control unit.
Many recording apparatus such as laser-beam printers have been disclosed in which a printer control unit having a high-speed data processing function is incorporated in addition to a printing unit which executes the actual printing on a medium. The unit that actuates the actual printing on the medium shall be referred to as a "printer unit", and the unit for controlling the latter shall be referred to as a "printer control unit".
In such a recording apparatus, the conventional practice generally is for the recording density to be fixedly set for each apparatus. When the printing recording density is to be changed, a serviceman makes the correction as by changing the rotational speed of a polygon motor.
Accordingly, the present applicant has disclosed art in which recording density is changed over in real-time using changeover means for changing the scanning timing of the laser beam.
In a recent printer control unit, vector font data sent from a host computer is converted into a raster image for the printer unit, and then the raster image is stored in its own random-access memory (hereinafter referred to as a "RAM"). This is a so-called postscript printer system. In order to make RAM capacity as small as possible in a control unit of this type for the purpose of reducing cost, a new raster image is developed in a RAM before changing over the printing recording density, without providing a raster-image RAM for every printing recording density of the recording unit.
A problem encountered is the point in time at which the change in recording density is to be made. The reason is that while the initiative for issuing the density changeover command resides with the printer control unit (and more particularly, with the host computer that sends the changeover command to the control unit), the activity required in order to change the recording density, e.g., the activity for changing the rotational speed of a polygon mirror, is carried out on the side of the printer unit.
If the serviceman performs the density changeover operation in the field, no problems arise because the printer unit will be off when the change is made.
However, if the recording density is changed in real-time as in the art proposed above, namely if the density changeover command is sent from the printer control unit side to the printer unit side and the operation for changing density is performed on the side of the printer unit based upon this command, the following problem arises:
If the recording density changeover is carried out within the printer unit during exposure, the rotational speed of the polygon motor changes and, as a matter of course, printing cannot be performed properly. Accordingly, the change in recording density must not be performed during exposure. This is the minimum requirement.
A major problem that arises is the following:
In a postscript-type printer control unit in which a raster image is developed in a RAM within the control unit, the time at which the raster image data in the RAM should be rewritten cannot be ascertained on the side of the printer control unit unless the latter is capable grasping correctly the timing at which the printing density changeover was performed on the side of the printer unit.
Accordingly, a technique has been conceived in which the recording paper is partitioned page by page and the printing density changeover timing is judged on the side of the control unit to be that at which transmission of each page of exposure data is completed.
However, if the printer unit is one capable of high-speed printing, another problem arises. Specifically, since the printer unit prints at high speed, several sheets of recording paper are present in the paper conveyance path of the printer unit. In a printer unit of this type, assume that the associated control unit has issued a 300 dpi.fwdarw.600 dpi density changeover command. Further, in a case where the timing for the changeover to 600 dpi is judged on the side of the control unit based upon the moment at which transmission of the first page of exposure data has been completed, and the second page of raster image data starts to be written in the RAM at 600 dpi from the moment of the judged timing, assume that a situation arises in which the first page of printing paper fails to be printed on for some reason, such as jamming of this sheet inside the printer unit. Ordinarily, in order to recover from jamming, the raster image of the first page must again be developed in the RAM at 300 dpi, which was the printing density originally intended for this page.
The fact that the data pertaining to the jammed page of recording paper must be re-developed in the RAM, i.e., the fact that the printing density must be returned to the original printing density, results in a major decline in efficiency. This can be fatal in a high-speed printer unit in which several sheets of recording paper can be present in the paper conveyance path at any time. In addition to returning to the original recording density when recovering from jamming, it becomes necessary to call the fonts for the original recording density. In particular, in a printer unit of the kind capable of changing over Chinese characters at various recording densities, generally the fonts corresponding to the printing density prior to the change are kept on, say, an external disk rather than in a RAM. In such case, therefore, one is compelled to rewrite all of the 600 dpi fonts in the RAM into 300 dpi fonts to effect the recovery, and then restore these fonts to the 600 dpi fonts. Such an operation is an impediment to an improvement in printing efficiency. The greater the distance of the paper conveyance path after the end of exposure, the greater the possibility that this problem will arise.
The foregoing problems are not limited to the postscript-type control unit but are encountered in all recording systems in which the control unit and printer unit are separated from each other.